Mammalian heme peroxidase enzymes are distributed in various tissues, but the common reactivity feature is utilization of hydrogen peroxide for oxidation of halide or pseudo-halide ions. Peroxidases in leucocytes and in the exocrine fluids generate oxidized halide or pseudo-halide species as a defense mechanism against invading microbes. Leucocyte myeloperoxidase oxidation of chloride ion may also be associated with an inflammatory response, and this oxidative process has been implicated in atherogenesis. Exocrine fluid peroxidases such as lactoperoxidas utilize thiocyanate ion as the physiological substrate. Only very recently hav the heme structures been identified for myeloperoxidase and lactoperoxidase. The structures are unprecedented for heme compounds, in that the 1- and 5-position methyl groups of the common protoporphyrin IX are converted to hydroxymethyl groups, and these moieties are esterified to glutamate and aspartate residues of the protein. The X-ray structure of myeloperoxidase reveals a third covalent linkage to the protein with sulfonium ion formation between a methionine residue and the 2-position vinyl group of the porphyrin. The fundamental reactivity properties of these unusual heme species merit investigation in terms of how the porphyrin substituents may dictate enzyme catalytic properties, and also in terms of known suicide substrate inhibition of the mammalian peroxidases. The specific objectives directed toward this long-term goal include the following: (1) chemical synthesis of metalloporphyrin complexes that contain either the hydroxymethyl or the vinyl sulfonium ion substituents, to include total synthesis of the heme isolated from lactoperoxidase; (2) physical characterization of the synthetic compounds with particular regard to substituents effects on redox potentials, and a correlation of the presence of the sulfonium ion linkage with chloride ion oxidation by myeloperoxidase; (3) investigation of the susceptibility of the synthetic compounds and the heme peptides of lactoperoxidase to modification b suicide substrates, with corresponding structural characterization of modified hemes; and (4) correlation of the unusual heme structures with formation of a unique, presumably antimicrobial oxidized thiocyanate intermediate.